1. Field of the Invention
This invention relates generally to a two-dimensional address or matrix device, and is more particularly directed to a two-dimensional display device employing liquid crystals.
2. Description of the Prior Art
It has been previously proposed to display a television picture on a liquid-crystal display device. Normally, such a device employs a plurality of picture element units disposed in an X-Y array or matrix, with each picture element unit being formed of a liquid crystal cell and a switching element, which can be an FET. Generally, the picture element units are arranged in n horizontal rows and m vertical columns. A horizontal scanning pulse generator, normally formed of a shift register, has m output terminals and cycles once for each horizontal line interval of an input video signal, so each of the m outputs is high for a fraction 1/m of the picture portion of a horizontal line interval. A vertical scanning pulse generator, normally formed as a shift register, has n output terminals, and cycles once each frame interval (i.e., odd output terminals are made high in turn during odd field intervals, and even output terminals are made high in turn during even field intervals).
Vertical signal transmitting lines are respectively connected to all of the n switching elements of each column, and horizontal signal transmitting lines are respectively connected to each of the m switching elements of each row. Each of the m vertical lines is connected to an output terminal of a respective input switching element, which has an input terminal connected to a signal input to receive a video input signal and has a control electrode connected to a respective one of the m output terminals of the horizontal scanning pulse generator. The n horizontal lines are each connected to a respective one of the n output terminals of the vertical scanning pulse generator.
At any given moment, the input video signal is applied to a single one of the picture element units, namely, that one for which the horizontal and vertical scanning pulses are both high. Each of the liquid crystal cells has a signal charge imparted to it, in turn, and the optical transmissivity of each such liquid crystal cell is governed by its respective signal charge.
A new signal charge is given to each liquid crystal cell during each video frame.
The liquid crystal display device so constructed presents a video picture formed of a mosaic of these cells, each having a particular optical transmissivity as governed by the level of the video signal at the time that the associated vertical and horizontal scanning pulses are both high.
In such a device, the shift register forming the horizontal scanning pulse generator requires m successive stages, and if good resolution is required the number m must be quite high. Consequently, the shift register can require a large amount of area if the circuit is formed as an integrated circuit (IC).
Also, since the period during which each input switching element is on can be only 1/m times the horizontal line interval, each liquid crystal cell has only a brief period in which to receive its respective signal charge. It has been discovered that this period is insufficient for the cells to accumulate enough charge for a good quality, high contrast television picture. Unfortunately, the amount of transferred signal charge cannot be increased merely by increasing the video signal voltage, because the structure of the liquid crystal cells will not tolerate application of higher voltages.
Also, when the accumulated signal charges in the liquid crystal cells are low compared with the corresponding values of the input video signal, any insufficiency in charging is likely to vary significantly from one picture element unit to the next, with a result that waveform distortion can occur in the television picture.
Further, if the resistance of the switching elements of the picture element units is reduced, i.e., if the transconductance thereof is made high where FETs are used for the switching elements, each such element is required to have a large gate width. Consequently, when formed as an IC, the chip area required for the display device can become excessively large.